There is known a welder for resistance butt welding of pipes, accommodated inside pipes to be welded (cf. U.S. Pat. No. 3,164,712 of Jan. 5, 1965).
The housing of this welder is a hollow bar whose longitudinal axis is matched with the axis of the pipes to be welded. Mounted on the housing in a coaxial arrangement are a welding transformer and mechanisms for clamping pipes to be welded, each being provided with a hydraulic drive to move the respective pipe clamping mechanism along the axis of the housing. The pipe clamping mechanisms are electrically connected to the welding transformer and located on its opposite sides. Also mounted on the housing is a pipe flashing and upsetting mechanism provided with a hydraulic drive of its own and kinematically coupled to one of the pipe clamping mechanisms. The housing also carries a hydraulic pump to supply working fluid to the hydraulic drives of the pipe clamping mechanisms and the pipe flashing and upsetting mechanism, as well as a mechanism for moving the welder along the axis of pipes to be welded.
The pipe clamping mechanisms of the welder under review are constructed as follows. The clip of each of the pipe clamping mechanisms is provided with slots to receive gripping jaws which are equidistantly arranged along the generatrix of the housing and driven out of their slots by the rod of a circular cylinder at the end of a cone-shaped ring. The gripping jaws come out of their slots as the cone-shaped ring contacts the chamfered internal surfaces of the jaws. The circular cylinder is made integral with the clip.
The foregoing design is disadvantageous in that the gripping jaws are frequently jammed by welding dribble or drippings which is unavoidable during resistance butt welding. Another disadvantage is that much of the force developed by the cylinder is lost because of the friction between the cone-shaped ring and the chamfered surfaces of the gripping jaws; as a result, the cylinder must be of a large size, although this makes it almost impossible to exclude leaks between the piston and the barrel of the cylinder. Finally, the cylinder is of the circular type, which necessitates sealing of the internal hole in the ring piston.
Much force is further lost because of the friction between the ring piston and its rod and the central bar. The manufacture of large-size circular cylinders with a number of seating surfaces is a labor-consuming and costly process.
The design of the clamping mechanism is such that after the gripping jaws are released they are brought back to the initial position only by gravity. On the other hand, it would be extremely difficult to build a special means for bringing the gripping jaws back to their initial position into a welder whose dimensions are restricted by the diameter of the pipe to be welded.
The pipe clamping mechanisms and the pipe flashing and upsetting mechanism of the welder under review all move with sliding friction on surfaces which are unavoidably exposed to dripping in the course of welding. The result is rapid wear and frequent jamming of these mechanisms.
The design of the clamping mechanism is such that the clips with the slots that receive the gripping jaws are unavoidably too close to the butt; this makes it impossible to arrange the welding transformer in the butt zone, which, in turn, accounts for an increased resistance of the welder's secondary circuit. The overall result is that welding is carried out at a high capacity and considerable power is consumed.
Finally, the welder incorporates high-accuracy, and is made of large-size parts. Naturally enough, such parts are difficult to manufacture.